1. Field of Invention
The present invention relates to methods and apparatus for locating coating faults on buried pipelines, and in particular to such methods and apparatus that rely upon magnetic field detection.
2. Description of Related Art
Many steps are taken to prevent buried metal pipe (e.g., steel pipe) from being subjected to corrosion by, for example, electro-chemical reactions. For example, it is known to coat the surface of the steel pipe with a material and/or to keep the pipe's electric potential equal or lower to that of the ground.
In many instances, it is necessary to regularly check the condition of the pipe after it is buried to determine whether any corrosion of the pipe has taken place. This is particularly important when the buried pipe is, for example, a gas pipeline.
Accordingly, it is known to check the condition of buried gas pipelines from above the ground using electromagnetic searching techniques. Using such techniques, the location of coating faults in the pipeline or the location where the pipeline touches another buried pipe, for example a water pipe, can be detected.
The publication entitled The 30th Automatic Control Annual Meeting (4044, pages 745-746, October 1987) describes a searching system that combines soil-to-soil potential methods with magnetic field measuring methods to locate coating faults. In particular, a magnetic field change that is generated due to a coating fault in a buried pipeline to which a base electric signal is applied is detected in order to locate the coating faults. This technique is prone to difficulties and mistakes because the soil-to-soil potential method requires that an electric signal is run from the road surface.
Japanese Laid-Open Patent Application No. 3-152411 discloses a technique that locates coating faults using only magnetic methods. However, with this arrangement, it is difficult to locate the origin of leakage current (from coating faults) because the magnetic field generated by the leakage current is weak and the disclosed method is strongly influenced from environmental noise.
Additionally, these techniques involve processing and recording the data by hand, which requires much time and complicated work.